Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system, a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. An EGR valve may be controlled to achieve a desired intake air dilution for the given engine operating conditions. Traditionally, the amount of low pressure EGR (LP-EGR) and/or high pressure EGR (HP-EGR) routed through the EGR system is measured and adjusted based on engine speed, engine temperature, and load during engine operation to maintain desirable combustion stability of the engine while providing emissions and fuel economy benefits. EGR effectively cools combustion chamber temperatures thereby reducing NOx formation. In hybrid vehicles, EGR may be continually provided each time the vehicle is propelled using engine torque. A diagnostic procedure may need to be periodically or opportunistically carried out to monitor operation of the EGR system.
Various approaches are provided for diagnostics of an EGR system. In one example, as shown in U.S. Pat. No. 5,508,926, Wade discloses a method for detecting restrictions in the EGR system during steady-state engine operations. Air pressure in the engine intake manifold is monitored over a test period while an amount (determined based on engine operating conditions) of EGR is delivered to the intake manifold. Changes in the monitored air pressure are filtered through a lag filter process comprising a dynamic filter coefficient. The filtered air pressure is then compared to a dynamic threshold to determine presence of a restriction in the EGR system.
However, the inventors herein have recognized potential issues with such systems. As one example, the approach of Wade may not be able to detect undesired EGR flow occurring when EGR is not requested. As another example, the approach of Wade is limited to steady-state conditions. In hybrid vehicles, during engine operation, EGR may be continually supplied and the EGR valve may be maintained in an open position, thereby providing insufficient time for carrying out diagnostics of an EGR valve stuck in an open position. Limited engine operation time in a hybrid vehicle may not be sufficient to gather a complete set of data indicative of degradation of the EGR valve.
In one example, the issues described above may be at least partly addressed by a method for an engine coupled to a hybrid vehicle comprising: upon receiving an engine shut-down request, before engine spin-down, rotating the engine at idling speed, indicating degradation of an exhaust gas recirculation (EGR) system based on a ratio of accumulated difference between a measured EGR flow and an EGR limit to accumulated intake air flow relative to a threshold; and adjusting EGR flow in subsequent engine cycles based on the indication of degradation . . . . In this way, by delaying engine shut-down and rotating the engine at an idling speed via an electric motor, a window may be provided for effectively carrying out EGR system diagnostics in a hybrid vehicle.
As one example, a diagnostic routine of the EGR system may be carried out periodically or opportunistically immediately after an engine shut-down request in a hybrid vehicle. In response to an engine shut-down request, a complete engine spin down may be delayed and the engine may be rotated at an idling speed (referred herein as virtual engine idling) to provide a window for carrying out EGR system diagnostics. The EGR valve may be commanded to a completely closed position and an actual EGR flow rate may be measured via an EGR flow sensor, such as a pressure sensor (either an absolute pressure sensor or a delta pressure sensor). A difference between a tolerance threshold and the measured EGR flow rate may be estimated to obtain an EGR mass flow error. Mass airflow via the intake manifold may also be estimated via a mass air flow sensor. The EGR mass flow error and the intake air mass flow may be accumulated over the duration of the virtual engine idling period. A complete dataset for the EGR system diagnostics may be obtained when the accumulated intake air mass flow reaches a threshold flow. If the dataset cannot be completed within one diagnostic window, the data may be saved in the controller memory and the diagnostics may be resumed during an immediately subsequent virtual engine idling window (during an engine shut-down request). Once a complete dataset has been recorded, the ratio of the accumulated EGR mass flow error to the accumulated intake air mass flow may then be compared to a threshold. Degradation of the EGR system may be indicated and a diagnostics code may be set if the ratio is higher than the threshold. In response to indication of EGR system degradation, EGR flow rate (opening of the EGR valve) may be adjusted during subsequent engine cycles to account for an undesired EGR flow. In one example, where the diagnostics approach is used to diagnose a high pressure (HP) EGR system, in response to degradation of the HP-EGR system, EGR may be delivered via a low pressure (LP) EGR system during subsequent engine operations.
In this way, by delaying engine spin-down in a hybrid vehicle, a window may be provided for closing the EGR valve and carrying out diagnostics of the EGR system. By carrying out EGR diagnostics with the EGR valve commanded to be closed, undesirable EGR flow caused by an opening in the EGR valve may be detected and distinguished from degradation of the EGR system resulting in insufficient EGR flow, and appropriately addressed. By estimating the undesired EGR flow based on an accumulated intake air flow, leaks in an EGR valve may be detected. The technical effect of continuing the EGR system diagnostics over a number of virtual engine idling windows is that a larger dataset may be accumulated to make the diagnostics routine robust. Overall, by enabling diagnostics of the EGR system to be carried out reliably and accurately in a hybrid vehicle, the fuel economy and emissions benefits of EGR may be extended during engine operations.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.